The present invention relates to instruments that measure soil water matric potential, and specifically to instruments known as hydraulic tensiometers. It relates also to the use of hydraulic tensiometers for the automatic, electrical control of irrigation systems.
Hydraulic tensiometers consist basically of a sealed, water-filled probe bounded at one end by a porous ceramic tip. The instruments can be permanently inserted into a soil or other growing medium to determine the need for irrigation. Depending on the matric water potential of the medium surrounding the ceramic tip, water will either be withdrawn from the tip to create a partial vacuum or negative pressure within the probe, or water will pass into the probe through the tip to increase the internal pressure. At equilibrium the pressure within the probe is equal to the matric water potential or `suction` potential of the surrounding medium. This is generally less than atmospheric pressure except when the soil is saturated, and is a direct measure of the energy required to extract moisture from the medium. Matric water potential is one of the principal parameters of soil moisture that govern movement of water through the soil-plant-atmosphere continuum. Monitoring of matric potential is therefore an appropriate means for determining the need for irrigation.
Hydraulic tensiometers can be very sensitive indicators of matric water potential when properly maintained. Maintenance functions include the venting of undissolved gases that tend to accumulate in such instruments over the course of time. It also involves the replenishment of an associated loss of instrument fluid (U.S. Pat. Nos. 2878671 and 3898872). Such maintenance functions conventionally require manual labor for, despite prior attempts (U.S. Pat. Nos. 2863698 and 2893641), no commercially successful self-servicing tensiometers are currently available to the author's knowledge.
Hydraulic tensiometers have been developed with electrical switching capabilities, and which are responsive to changes in soil matric water potential (U.S. Pat. Nos. 3559062, 3806851, and 3910300). When used with automatic irrigation systems, however, such instruments tend to prematurely terminate irrigation. This arises from an overreaction to the first arrival of moisture at the sensor location. The result is an isolation of the ceramic tip at an artificial boundary between wet and dry soil. The consequence is excessive and frequent irrigations to the depth of the sensor only. Conversely, a poorly serviced hydraulic tensiometer with an air void will exhibit a slow and indeterminate response to the arrival of irrigation water.
Thus it can be appreciated that there exists a need for a self-servicing hydraulic tensiometer with electrical switching capabilities, and which exhibits high sensitivity during the drying phase and a lesser, but defined sensitivity during the wetting phase. A novel approach to delaying the response during wetting is to induce a hydraulic tensiometer to absorb a significantly greater and definable volume of water during wetting than that which was lost during drying. An instrument that achieves this and other objectives is the subject of this patent disclosure.